30-day weather outlook for September 3, 2018 – October 3, 2018
Summary- Strong upper high pressure produces dry and hot conditions in California during Sept 7-12th.
Trough with chance for showers occurring on Sept 15-16th. Coolest dates currently are 15-16th, then the trough weakens on 17th.
Warmer than normal sea surface will continue off the coast of central and S California and western Mexico. Sea surface temperatures on the evening Sep 3 were 70 or 68 Ventura Co coast, to mid-70s San Diego Co coast, and 67 at Tanner Bank Buoy well W of San Clemente Island.
Although less hot than in the recent past, dry, breezy conditions with continued high risk of wildland fires will continue.
After a warm or hot spell during 28-22nd, there is possible return to cooler and showery conditions in the NORCAL coast Siskiyou’s, Lake Shasta, and Plateau during Sept 27-30th and Oct 1.
The El Niño (warmer than normal sea surface) along the equatorial central and east Pacific, continues moderate through the fall. The time of normal maximum intensity of El Niño is generally Dec through Feb. Sea surface temperature anomalies (SSTA) remain high through the fall and into early winter, with a warmer than normal fall and early winter suggested in the CFSv2 model guidance.
The sea surface temperature anomaly pattern off South America at 30-45S supports a possibly aggressive frost period this early to mid-spring in Chile’s viticultural areas, from Santiago south. This includes Vicuna, Elke Valley Valparaiso and the intermediate valleys (Bernardo-O-Higgens area). This risk includes the more critical period…spring blooms for winegrapes in late Sept and all of October.
In the Near Term – Sept 8-22 – Sept 15…Salinas Valley-San Luis Obispo Co…After hot days from Sept 6-11th, troughing and a cool period returns 12-17th. It turns warmer again on the 18-22nd.
…S Calif Avocado Area, San Luis Obispo Co to San Diego Co…Sea surface temperatures along the SOCAL coast have continued unusually warm, with 70s for the most part near San Diego, and upper 60s in Ventura Co. Leftover tropical moisture from tropical cyclones may occasionally drift into SE California/San Diego/Imperial Co’s and S Deserts with some monsoonal showers and thunderstorms (TSTMS).
Summary – Sept 22 – October 4… In the southern California avocado growing areas, from San Luis Obispo south, it turns hot inland from Sept 22-26th. Cooler from the 27-30th, and continuing into the first week or so of Oct, per CFSDailyAI forecast.
Seasonal Outlook / El Niño Update...Oct – Dec… It appears that the winter rains may be slow to arrive in Oct – Dec in central and N California. Above normal temperatures will be predominant, suggesting continued high snow levels. This makes logical sense due to the expected warmer than normal sea surface off to the southwest of NORCAL.
El Niño-associated warming of the equatorial sea surface in the central and east Pacific to near Peru/Ecuador is occurring and will continue.
The El Niño typically adds extra energy to drive the subtropical westerlies during the winter rain season.
The effect of El Niño to drive the subtropical westerlies can start early, e.g Oct. In the early part of the season. This year, the beginnings of El Niño will tend to assist development of troughs and upper lows along the California coast. Stronger than normal cutoff lows may start to develop in Oct near S California in the period from Oct 10-16th. Because the sea surface this year is significantly warmer (+4 F) than normal off Baja and around the SOCAL coast, some cutoff lows have greater potential to produce significant rains during the early season in Oct.
For November there is a trend towards the return of very warm and dry conditions, per the latest CFSv2 guidance.
For S California, from Oct – Dec…Near or above normal rainfall is currently suggested for Oct as upper lows and fronts tap into the abundant moisture available to the SW of S California, and continued warm sea surface along the SOCAL coast. The probability of rain from upper lows appears to be greater in Oct than Sept.
Usually we see a Santa Ana or two in Oct. The chance for Santa Ana-like winds is greatest for N California, and less for SOCAL for this Oct. However, with expected El Niño forcing, the chances for Santa Anas currently appear lower during this Oct.
After Oct, the next opportunity for rains in SOCAL should be in mid-Nov. Dec currently looks mostly dry in SOCAL, while N California receives rains.
...Alan Fox, Fox Weather LLC...